Motor or ESC or both?

After sitting in my living room for a few weeks, I took my P1 out for a flight today only to find that one of the motors is not spinning. It seems to oscillate back and forth rapidly. I can't imagine how it went bad just sitting in my living room. I guess there could be two problems. Either the motor or the ESC. Does anybody have any ideas as to what part to replace?

This is sounding like what commonly happens when a motor coil is damaged but it can be an ESC as well.

Have you ever installed prop-guards?
It so happens that the screws for use WITH the guards are longer than the stock or originals. People remove the guards, replace the screws and because they are longer enter and damage the motor. This damage can often be seen when you remove the motor and look down the mounting holes to the coils.

Having said that there is not much that can go wrong with the motors other than damage from crash or the wrong screws.

Do you have soldering skills?
You can swap the motor with a known good or operating one to see if the problem follows the motor or not. This will determine if it is the motor, ESC, or even possibly both.

What's weird is that the copter has been running fine for quite a while, even with the prop guards. It's been sitting in my living room for a few weeks since last flight when I decided to take it out for a flight. That's when I realized I had this problem. Where is a good place to buy the right motor/ESC for the copter? I don't mind buying both but have noticed there is an ESC red and an ESC green. I have no idea which to purchase. Maybe I should send the copter into DJI for repair. I'm just not sure what to do. And yes I do have extensive soldering experience.

As N017RW suggests try swapping a motor if it's not then might be the ESC .
Are the lights working on that arm ? Have you popped the shell and checked all the wiring ? Do you have kids that might have played with it ?